My gut tells me you should check that all the lifters are in their bore. (shrug)
Your symptoms are kinda opposite a lifter floating up and down the bore, till it got revved enuff to pop it right out the bore.
If a lifter popped out of a bore, I'd be inclined to think it's run like crap. Runs like a stalled dog when It has pressure and I can actually drive it. This problem has been on and off for a bit. Maybe the last 400 miles or so.
